Optical-interface patterning for radiation detector crystals

ABSTRACT

A radiation detector is disclosed that includes a scintillation crystal and a plurality of photodetectors positioned to detect low-energy scintillation photons generated within the scintillation crystal. The scintillation crystals are processed using subsurface laser engraving to generate point-like defects within the crystal to alter the path of the scintillation photons. In one embodiment, the defects define a plurality of boundaries within a monolithic crystal to delineate individual detector elements. In another embodiment, the defects define a depth-of-interaction boundary that varies longitudinally to vary the amount of light shared by neighboring portions of the crystal. In another embodiment the defects are evenly distributed to reduce the lateral spread of light from a scintillation event. Two or more of these different aspects may be combined in a single scintillation crystal. Additionally, or alternatively, similar SSLE defects may be produced in other light-guiding elements of the radiation detector.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/255,407, filed Oct. 27, 2009, which is hereby incorporated byreference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under EB002117 awardedby National Institutes of Health, National Institute of BiomedicalImaging and Bioengineering. The Government has certain rights in theinvention.

BACKGROUND

Scintillation crystal radiation detection systems rely on high-energyphotons, such as gamma rays, interacting with a scintillation materialin a Compton scattering or photoelectric interaction. The scintillationevent produces a large number of lower-energy photons that are morereadily detected using a photodetector, for example, a photomultipliertube, silicon photomultiplier, or the like.

Exemplary scintillation crystals include NaI(TI) (thallium-doped sodiumiodide), BGO (bismuth germinate), LSO (lutetium oxyorthosilicate), GSO(gadolinium orthosilicate), LYSO (cerium-doped lutetium yttriumorthosilicate), LuAP (lutetium aluminum perovskite), LGSO(Lu0.4Gd1.6Si05: 22.0 mol % Ce), LaBr3 (lanthanum bromide) and the like.For example, when BGO interacts with high-energy radiation, such asgamma-rays or x-rays, it emits a green fluorescent light with a peakwavelength of 480 nm. BGO is used for a wide range of applications inhigh-energy physics, nuclear physics, space physics, nuclear medicine,geological prospecting, and other industries. LYSO crystal has theadvantages of high light output and density, quick decay time, excellentenergy resolution, and moderate cost. These properties make LYSO a goodcandidate for a range of detection applications in nuclear physics andnuclear medicine, which require improved timing and energy resolution.

In typical scintillation crystals used in positron emission tomography(PET), for example, an incident gamma photon having a nominal energy of511 keV interacts in the scintillation crystal to produce tens ofthousands of low-energy (e.g., visible wavelength) photons (˜1 eV) in avery short flash or scintillation event. The number of scintillationphotons produced in the crystal is proportional to the energy depositedby the photon.

The lower-energy scintillation photons are then detected withphotodetectors that are typically placed in an array on one side, or onopposite sides of the scintillation crystal. Typical photodetectorsinclude photomultiplier tubes (PMT), avalanche photodiodes (APDs),Si-PIN photodiodes, silicon drift photodiodes, and siliconphotomultipliers (SiPM). The radiation detector is configured toidentify a high-energy photon interaction by detecting the low-energyphotons produced in the scintillation event, and to determine thelocation of the scintillation event within the scintillation crystal(preferably, in three spatial dimensions), the time of the scintillationevent, and the total energy of the event.

Positron Emission Tomography (PET)

Although radiation detectors in accordance with the present inventionare contemplated to have applications in several different fields,ranging from cosmological imaging to the detection of radioactivematerials, a particular application in the field of medical imaging willbe described in some detail to assist the reader in understanding thedescription of the invention that will follow.

Positron emission tomography (PET) is a medical imaging modality thattakes advantage of radioactive decay to measure metabolic activitiesinside a living organism. A PET imaging system comprises three maincomponents, indicated schematically in FIG. 1, a radioactive tracer thatis administered to the subject to be scanned, a scanner that is operableto detect the location of radioactive tracer (indirectly as discussedbelow), and a tomographic image processing system.

The first step is to produce and administer a radioactive tracer 90,comprising a radioactive isotope and a metabolically active molecule.The tracer 90 is injected into the body to be scanned 91. After allowingtime for the tracer 90 to concentrate in certain tissues, the body 91 issuitably positioned inside the scanner 92. The radioactive decay eventfor tracers used in PET studies is positron emission. An emittedpositron travels a short distance in the body tissue until it interactswith an electron. The positron-electron interaction is an annihilationevent that produces two 511 KeV anti-parallel photons. The scanner 92 isadapted to detect the pair of photons from the annihilation eventsimultaneously.

The scanner 92, the second component of PET system, includes a ring ofsensors that detect the 511 KeV photons, and front-end electronics thatprocess the signals generated by the sensors. As discussed above, thescintillators 93 converts the 511 KeV high-energy photons into manylower-energy photons, typically visible light photons. Thephotodetectors (e.g., PMT, SiMP or APD) 94 detect the visible lightphotons and generate a corresponding electrical pulse. The pulses areprocessed by front-end electronics to determine the parameters orcharacteristics of the pulse (e.g., energy, timing). The front-endelectronics may include, for example, one or more low-pass filters 96,analog-to-digital converters 97, and field programmable gate arrays 98.The sensors typically comprise scintillators 93 and photodetectors 94.

The data is sent from the front-end electronics to a host computer 95that performs tomographic image reconstruction to turn the data into a3-D image.

A 511 KeV photon has a substantial amount of energy and will passthrough many materials, including body tissue. While this typicallyallows the photon to travel through and exit the body, the high-energyphotons are difficult to detect. Photon detection is the task of thescintillator 93. The scintillator 93 absorbs or otherwise interacts withhigh-energy photons and emits a relatively large number of lower-energyphotons, typically visible light photons. The scintillator 93 can bemade from various materials, including plastics, organic and inorganiccrystals, and organic liquids. Each type of scintillator has a differentdensity, index of refraction, timing characteristics, and wavelength ofmaximum emission. For convenience, in the present applicationscintillators will sometimes be referred to as “crystals,” althoughother suitable scintillator materials are also contemplated. Forpurposes of this application “scintillator crystals” is defined toencompass any suitable scintillation material.

In general, the density of the scintillator determines how well thematerial stops the high-energy photons. The index of refraction of thescintillator crystal and the wavelength of the emitted light affect howeasily light can be collected from the crystal. The wavelength of theemitted light also needs to be matched with the device that will turnthe light into an electrical pulse (e.g., the PMT) in order to optimizethe efficiency. The scintillator timing characteristics determine howlong it takes the visible light to reach its maximum output (rise time)and how long it takes to decay (decay time). The rise and decay timesare important because the longer the sum of these two times, the lowerthe number of events a detector can handle in a given period, and thusthe longer the scan will take to get the same number of counts. Also,the longer the timing characteristics, the greater the likelihood thattwo events will overlap (pile-up), which can result in lost data.

Attached to the scintillator 93 are electronic devices that convert thevisible light photons from the scintillator 93 into electronic pulses. APMT is a vacuum tube with a photocathode, dynodes, and an anode that hashigh gains to allow very low levels of light to be detected. APDs are asemiconductor version of the PMT, but with significantly lower gaincharacteristics. SiPMs comprise an array of semiconducting photodiodesthat operate in Geiger mode so that when a photon interacts andgenerates a carrier, a short pulse of current is generated. In anexemplary SiPM, the array of photodiodes comprises about 103 diodes permm². All of the diodes are connected to a common silicon substrate sothe output of the array is a sum of the output of all of the diodes. Theoutput can therefore range from a minimum wherein one photodiode firesto a maximum wherein all of the photodiodes fire. This gives thesesdevices a linear output even though they are made up of digital devices.

In conventional PET detectors the scintillators 93 comprise discretecrystals arranged in a two-dimensional planar array, and then arrangedinto a ring as shown in FIG. 1. The photodetectors 94 for detecting theflashes of scintillation light are typically positioned adjacent theback surface of each individual crystal. The signals from thephotodetectors are analyzed to estimate the x-y location of thescintillation event, to estimate the depth of interaction (i.e.,z-location), to determine the time of the interaction, and to estimatethe total energy deposited in the scintillator. However, given the smallcrystal cross-sections required to obtain very high resolution, discretecrystal designs are typically expensive, have low packing fraction,reduced light collection, and are labor intensive to build.

The present inventors have researched and developed new and advanceddetectors for PET scanners. For example, cMiCE: a high resolution animalPET using continuous LSO with a statistics based positioning scheme, J.Joung, R. S. Miyaoka, T. K. Lewellen, Nuclear Instruments & Methods inPhysics Research A 489 (2002) 584-598 (Elsevier), which is herebyincorporated by reference in its entirety, a continuous miniaturecrystal element (cMiCE) detector for small animal scanners is discussed.See also, U.S. Patent Application Publication No. 2010/0044571, which isalso hereby incorporated by reference in its entirety.

In another example, New Directions for dMiCE—Depth-of-InteractionDetector Design for PET Scanners, T. K. Lewellen et al., IEEE Nucl SciSymp Conf Rec (2007); 5:3798-3802, which is hereby incorporated byreference in its entirety, a novel depth-of-interaction (DOI) detectordesign based on light sharing between pairs or quadlets of crystals isdiscussed. See also, PCT Application Publication No. WO 2010/048363,which is also hereby incorporated by reference in its entirety.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

A radiation detector is disclosed having a transparent scintillatorblock, which is modified using subsurface laser engraving to produce aplurality of internal defects within the block. The internal defects areprecisely configured to alter the path of the low-energy scintillationphotons. A plurality of photodetectors configured to detect thescintillation photons from scintillation events occurring in the block,and to generate an output signal. A computer system is configured toreceive the output signals from the photodetectors for calculating thelocation of the scintillation event.

In an embodiment of the radiation detector the scintillation block isformed from thallium-doped sodium iodide, bismuth germinate, lutetiumoxyorthosilicate, gadolinium orthosilicate, or cerium-doped lutetiumyttrium.

In an embodiment of the radiation detector the plurality ofphotodetectors are photomultiplier tubes, avalanche photodiodes, orsilicon photomultipliers.

In an embodiment of the radiation detector the scintillator block is amonolithic crystal, and the internal defects define a plurality ofreflective walls that define boundaries segmenting the monolithiccrystal into a rectangular array of scintillator elements.

In an embodiment of the radiation detector the scintillator block is amonolithic crystal with the array of photodetectors disposed on a firstface of the crystal, and further the internal defects are distributedevenly throughout the crystal.

In an embodiment of the radiation detector the scintillator blockdefines a depth-of-interaction detector comprising a first portion witha photodetectors at a first end of the crystal element and a secondportion with a second photodetectors at the first end of the crystalelement, and the plurality of internal defects are disposed along atransverse plane that delineates the first portion of thedepth-of-interaction detector from the section portion. The internaldefects may define a triangular barrier and/or may be varied in densityin the longitudinal direction.

In an embodiment of the radiation detector, a transparent light guide isdisposed between the scintillator block and the plurality ofphotodetectors.

A scintillator for a radiation detector is disclosed, comprising amonolithic block of scintillation material having a plurality ofsubsurface laser engraved defects configured to interact withvisible-wavelength photons produced in a scintillation event occurringin the scintillator.

In an embodiment of the scintillator the plurality of subsurface laserengraved defects define a grid of internal optical barriers that extendthrough a thickness of the scintillator, dividing the scintillator intoa regular array of rectangular scintillator elements.

In an embodiment of the scintillator, the plurality of subsurface laserengraved defects define a depth-of-interaction optical boundary for eachof the scintillator elements that extends partially through thescintillator element along a plane, wherein the depth-of-interactionoptical boundary is triangular.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is an environmental view showing a PET scanner system;

FIG. 2 is a perspective view of a monolithic scintillation blocksegmented into an eight by eight array of elements by a plurality ofsubsurface laser engraved walls or barriers that define reflectiveinternal surfaces;

FIG. 3 is a perspective view of a crystal element from the monolithicscintillation block shown in FIG. 2, and showing diagrammatically ascintillation event;

FIG. 4 is a perspective view of a depth-of-interaction monolithiccrystal pair with an internal reflective barrier or feature formed bysubsurface laser engraving;

FIG. 5 is a perspective view of a depth-of-interaction monolithiccrystal pair wherein an internal reflective feature is formed withgraded density by subsurface laser engraving; and

FIG. 6 is a perspective view of a scintillation detector with acontinuous monolithic scintillation crystal having distributedpoint-like defects through the volume of the crystal that are formed bysubsurface laser engraving.

DETAILED DESCRIPTION

We propose a novel application for subsurface laser engraving (SSLE),wherein SSLE is used to produce point-like defects in interior regionsof an optical element such as a scintillation crystal. The defects areconfigured to control the transmission of low-energy photons within theoptical device. For example, and as discussed in more detail below, anarray of point defects produced by SSLE can serve as a reflectiveoptical boundary or interface in an optical element, such as ascintillator, light guide, or lens. The pattern and density of the pointdefects and the characteristics of individual point defects determinedby the SSLE process can be used to control the transport and/ordistribution of light in the optical element. Furthermore, theseproperties of the point-defect pattern can vary as a function ofposition to make light transport non-stationary over the interface.Alternatively or in addition, a distribution of point defects introducedthrough the volume of an optical element can be used to reduce thelateral spread of the flash of scintillation photons. The defectsproduced by SSLE in the optical element may be produced to provide highefficiency reflection of photons, such that the light may be redirectedin the optical element with relatively low loss (absorption) of photons.

Subsurface laser engraving is the process of creation or generation oftwo-dimensional or three-dimensional patterns within an optically clearmaterial (e.g., glass, crystal or plastic) using a focused laser. Thetransparency of the material minimizes the attenuation and distortion ofthe laser. The lateral position of a pulsed laser beam is typicallycontrolled by pairs of coordinated mirrors or right prisms (see, forexample, U.S. Pat. No. 7,371,596, which is hereby incorporated byreference in its entirety). In SSLE, the depth of focus may becontrolled by an actuated focusing element. Energy deposition by thepulsed laser at the focal spot causes rapid local heating, melting,and/or stress, which results in a relatively small or point-like defect,for example, a micro crack. Material properties at the point-like defectsuch as dopant diffusion, oxidation, crystallization, grain boundaries,fracturing, and so forth may be altered from that of the surroundingbulk material.

With careful generation and placement of the defects created by SSLE,the defects may be spatially constrained even in brittle scintillationcrystals. For example, if the size of the defect is small compared tothe distance to the boundary or another defect, then stress-inducedfractures will not propagate far beyond the heated volume. The defectsize depends on the size of the focal spot, on the rate of energydeposition and diffusion, and on the laser wavelength. In this manner,SSLE has been used to produce images in optically clear materials to beused as souvenirs or promotional items. A general process of SSLE isdisclosed in U.S. Pat. No. 5,206,496, which is hereby incorporated byreference in its entirety. See also U.S. Pat. Nos. 6,969,820; 6,426,480;and 4,843,207.

Monolithic Scintillator Block with Internal Optical Boundaries DefinedUsing SSLE

In a first embodiment shown in FIG. 2, a scintillator block 100 isformed from a monolithic block of scintillation material, for example, aLYSO crystal measuring 49.6 mm×49.6 mm×15 mm. A grid of optical barriers102 are formed through the block 100, each barrier formed as a planararray of defects etched using SSLE. As shown in FIG. 2, in thisembodiment the optical barriers 102 are perpendicular to the upper andlower faces of the block 100, and are configured to define a regulararray of 64 transparent crystal elements 104. Each of the definedcrystal scintillator elements 104 are approximately 3.1 mm×3.1 mm×15 mm.

Therefore, the scintillator block 100 defines a square 8×8 array ofscintillator elements 104, having substantially reflective boundariesdefined by the SSLE-generated optical barriers 102 therebetween. Inaddition, the outer perimeter wall 106 of the block 100 is typicallyprovided with a reflective barrier, which may be formed, for example, bylaser engraving the surface of the block 100, or by conventionalapplication of a reflective material. Although a square 8×8 array isshown in this exemplary embodiment, it will be appreciated that otherarray sizes may alternatively be used, including, for example, a 16×16array, an 8×16 array, non-square arrays, etc. Also, although uniformrectangular scintillator elements 104 are shown, it will be readilyapparent that with SSLE it is straightforward and contemplated by thepresent invention that the scintillator elements may be advantageous insome situations to define an array of scintillator elements havingdifferent sizes in an array, and/or with different shapes, including,for example, trapezoidal elements.

Advantageously, the scintillator block 100 comprises a single unitaryblock, rather than a conventional assembly of individual elements.Because of the barriers 102 defined within the block 100, the crystalelements 104 act substantially as separate scintillator blocks. FIG. 3illustrates a fragmentary portion of the block 100, including anindividual crystal element 104. A gamma ray 80 enters the crystalelement 104 and interacts by either Compton scattering or photoelectricinteraction to produce an omnidirectional flash of scintillation photons82. Some of the photons will be directed towards an upper face 105U or alower face 105L of the crystal element 104, to be detected by aphotodetector (not shown). Most of the photons having a more lateraltrajectory will reflect one or more times from the optical boundary 102and eventually exit the crystal element 104 from either the upper face105U or the lower face 105L. It is contemplated that some of the photonsmay pass through gaps in the optical boundary 102 and reflect fromoptical boundaries 102 in neighboring crystal elements 104, to bedetected by neighboring photodetectors. In such cases, the signals frommultiple neighboring photodetectors may be analyzed collectively tocalculate the position, timing, and energy of a particular scintillationevent.

The disclosed construction of the scintillator block 100 providesseveral advantages over the prior art. For example, only a single blockof scintillation material (e.g., LYSO) must be fabricated to produce aplurality of crystal elements 104, greatly reducing the time and expenseassociated with manufacturing and assembling a large number separateblocks. In addition, no additional light blocking or light reflectingmaterial need be provided and assembled between separate blocks.

The disclosed construction also eliminates the dead zones created byreflective materials disposed between individual blocks in prior artconstructions, which do not produce scintillation photons. For example,in prior art scintillation blocks formed from a plurality of individualblocks assembled with a reflective material therebetween, the reflectivematerial is not a scintillation material, and would not produce flashesof light (scintillation photons) if a gamma ray, for example, interactswith the reflective material. Gamma rays that interact with thereflective material would therefore not be detected. As the design ofradiation detectors move towards smaller individual scintillator blocksizes (in order to increase image resolution) the dead regions definedby the reflective material will become more significant. In the block100 defined above, however, the entire block is LYSO, and will thereforeproduce a flash of light when a Compton or photoelectric interactionwith a gamma ray 80 occurs.

Depth of Interaction Detector Design

In New Directions for dMiCE—Depth-of-Interaction Detector Design for PETScanners referenced above, a depth-of-interaction scintillation detectoris disclosed wherein a pair of scintillation crystals are arrangedside-by-side, and a shaped reflector is provided covering a portion ofthe interface between the two scintillation crystals. The reflector maybe triangular shaped, for example. As discussed in detail in thereferenced paper, the paired crystals have a transparent regiontherebetween, and will therefore “share” a portion of the light producedin a scintillation event in either of the crystals. Photodetectors areprovided for at least one end of both crystals, and the amount of lightdetected for both crystals can be analyzed. By comparing the detectedsignals from both crystals, an estimate depth-of-interaction (thelongitudinal position of the scintillation event) can be calculated. Inprior embodiments of this depth-of-interaction detector, the twocrystals and the reflector are formed separately, and the threecomponents are assembled with an adhesive.

FIG. 4 illustrates an exemplary depth-of-interaction detector 110,constructed in accordance with the present invention, and comprising amonolithic crystal 111. In this embodiment, the monolithic crystal 111is processed using SSLE to define an internal boundary 112 comprising anarray of spaced point-like defects generally along a transverse plane. Apair of photodetectors 116 are provided adjacent a bottom face 115 ofthe crystal 111. The internal boundary 112 is generally triangular,extending across the width of the crystal at the bottom face 115 andtapering as it extends upwardly most of the way to the top of thecrystal 111. Although a triangular boundary 112 is shown, other shapeshaving a portion that extend only part of the transverse plane may bealternatively used, and are contemplated.

It will be appreciated that the disclosed configuration is simpler toconstruct than similar prior art depth-of-interaction crystal pairs thatare assembled from two separate crystals. Therefore, this constructionshares the advantages discussed above. An additional advantage for thistype of depth-of-interaction monolithic crystal is that the non-blockedportion of the crystal along the plane defined by the triangularboundary 112 does not include a joint or interface between two separatecrystals. Therefore, the present construction will improve thelight-sharing between the two sides of the crystal 111.

FIG. 5 illustrates another embodiment of an exemplarydepth-of-interaction paired crystal detector 120 with a monolithiccrystal 121. The crystal 121 is processed using SSLE to define areflective internal boundary 122. In this embodiment the internalboundary 122 extends across the width of the crystal 121 for its entirelength, rather than having a tapering shape. However, the density of thepoint-like defects that define the reflective boundary 122 varies in thelongitudinal direction. In a particular embodiment, the density of thedefects decreases continuously from the base of the boundary 122, suchthat the density of the defects near the photodetectors 116 at the face125 of the crystal is much greater. Therefore, the amount of light thatis shared between the two portions of the crystal 121 (the portions oneither side of the internal boundary 122) varies along the length of thecrystal 121. The depth of a scintillation event occurring in thedetector 120 may therefore be estimated by comparing the light signalresponses from the two photodetectors 116.

It is contemplated, however, that the concentration of defects may varyin a more complicated manner, for example, with a minimum defect densityat an intermediate point along the length of the crystal. It will alsobe apparent from the present disclosure that the density of the defectsmay be varied on a shaped internal boundary such as that shown in FIG.4.

Continuous Crystal without Discrete Barriers

In another embodiment of the present invention, a scintillation crystalis formed without reflective barriers that define separate crystalelements. As discussed in cMiCE: a high resolution animal PET usingcontinuous LSO with a statistics based positioning scheme, and in U.S.Patent Application Publication No. 2010/0044571, referenced above, acontinuous miniature crystal element (cMiCE) detector may be formed as asingle slab of scintillator material (for example, LYSO), and thesignals from an array of photodetectors on a face of the continuouscrystal may be analyzed to estimate the three-dimensional location of ascintillation event within the crystal.

For example, some of the present inventors have developed a continuousminiature crystal element detector composed of a 50-mm×50-mm×8-mm slabof LYSO, coupled with a 64-channel, multi-anode, flat-panelphotomultiplier tube. A statistics-based positioning algorithm analyzesthe light response function from the array of photomultiplier tubechannels to calculate the three-dimensional position of a scintillationevent within the crystal.

A diagrammatic side view of a continuous crystal photon detector 130 inaccordance with the present invention is shown in FIG. 6. In thisembodiment a monolithic scintillating crystal 131 is shown with a pairof photosensor entrance windows or light guides 138 provided on oppositesides of the crystal 131, and a two-dimensional array of photodetectors136 disposed on each of the light guides 138. The scintillating crystal131 is configured with a plurality of point-like defects 132 that areproduced by SSLE and distributed throughout the crystal 131. In thecurrently preferred embodiment the defects 132 are approximatelyuniformly distributed throughout the crystal, although it iscontemplated there may be advantages to having non-uniformly distributeddefects. For example, boundary issues may be mitigated by providing amore dense distribution of defects near the lateral boundaries of thescintillating crystal 131, e.g., to reduce the number of photons thatescape without encountering one of the photodetectors 136.

Although in this embodiment separate photodetectors 136 are disposed ontwo opposite faces of the crystal 131, it is contemplated that thepresent invention may alternatively be practiced with differentphotodetector configurations. For example, an array of photodetectors136 may be provided on only the entrance surface (or the surfaceopposite the entrance surface) of the crystal 131, with the opposingface roughed or otherwise configured to reflect photons. In anotherexample, photodetectors may be provided on the lateral faces of thecrystal 131 to capture information from photons escaping from theperimeter of the crystal 131.

Also diagrammatically illustrated in FIG. 6 is a gamma ray 80 thatenters the crystal 131 and produces a scintillation event (at 81) thatproduces a large plurality of visible-wavelength scintillation photons84 that are generally distributed uniformly in all directions. Asillustrated, many of the scintillation photons 84 will reflect fromdefects 132 in the scintillation crystal 131 before exiting the crystal131. The crystal 131 has a lateral dimension that is greater than itsthickness, and therefore on average the scintillation photons 84 mayencounter more defects 132 as they travel laterally through the crystal131. The lateral spread of the light will therefore be preferentiallyattenuated by defects distributed throughout the crystal 131.

The distributed defects 132 will tend to sharpen the signal detected bythe photosensors 136 from the scintillation photons 84, as illustratedby the graph associated above the detector 130. In the graph, the curve70 illustrates diagrammatically a light distribution probability densityfunction (PDF) for the scintillation event 81, as compared qualitativelywith the light distribution PDF 72 that would be produced in a similardetector without the defects 132. It will be appreciated that thesharper PDF 70 resulting from the distributed defects in the crystalwill increase the accuracy of the calculated three-dimensionalestimation of the scintillation event location 81.

It will also be appreciated that although the point-like defects shownin FIG. 6 are disposed in the scintillation crystal 131, other opticalelements in a scintillation detector may be similarly provided withpoint-like defects using SSLE to achieve a desired control of thescintillation light. For example, the light guides 138 may be fabricatedwith a distribution of defects to advantageously direct thescintillation light. In other embodiment of scintillation detectors,light guides and/or optical fibers are used to connect the scintillationcrystal with the photodetectors. It is contemplated that such opticallight guides may similarly be provided with defects to control and/ordirect the scintillation light.

The examples described above and shown in the accompanying figuresillustrate several embodiments for controlling the light output from ascintillation material to facilitate and improve detection of lightresulting from scintillation events. As discussed above, the presentmethod and devices provide advantages of improved performance, reducedcost of production, and simplified production.

Also, while different aspects of the present invention are disclosed inseparate embodiments, it is contemplated that the novel aspects maybecombined in single apparatus. For example, it is contemplated that SSLEmay be used to configure a monolithic crystal with optical barriers,such as barriers 102 shown in FIG. 2, and with partial barriers such asthe boundaries 112 shown in FIG. 4, such that the crystal defines anarray of depth-of-interaction crystal portions. Alternatively, or inaddition, the crystal portions may be provided with a smoothlydistributed plurality of internal defects such as the defects 132 shownin FIG. 6.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

1. A scintillation crystal-type radiation detector comprising: atransparent scintillator block formed from a scintillation material thatis configured to interact with a high-energy photon in a scintillationevent that releases a large number of low-energy photons; and aplurality of photodetectors positioned to receive at least some of thelow-energy photons from the scintillation event and to producecorresponding output signals; a computer system configured to receivethe output signals from the plurality of photodetectors and to calculatethe location of the scintillation event within the scintillator block;wherein the scintillator block includes a plurality of internal defectscreated in the scintillator block using subsurface laser engraving, andwherein the plurality of internal defects are configured to alter thepath of at least some of the large number of low-energy photons withinthe scintillator block.
 2. The radiation detector of claim 1, whereinthe scintillation material comprises one of thallium doped sodiumiodide, bismuth germinate, lutetium oxyorthosilicate, gadoliniumorthosilicate, and cerium-doped lutetium yttrium orthosilicate.
 3. Theradiation detector of claim 1, wherein the plurality of photodetectorscomprise components selected from photomultiplier tubes, avalanchephotodiodes, and silicon photomultipliers.
 4. The radiation detector ofclaim 1, wherein the transparent scintillator block comprises amonolithic crystal, and further wherein the plurality of internaldefects define a plurality of reflective walls that define boundariessegmenting the monolithic crystal into a rectangular array ofscintillator elements.
 5. The radiation detector of claim 1, wherein thetransparent scintillator block comprises a monolithic crystal and theplurality of photodetectors are disposed on a first face of the crystal,and further wherein the plurality of internal defects are distributedevenly throughout the crystal.
 6. The radiation detector of claim 1,wherein the plurality of internal defects are evenly distributedthroughout the scintillator block.
 7. The radiation detector of claim 1,wherein the scintillator block comprises at least onedepth-of-interaction crystal element comprising a first portion havingone of the plurality of photodetectors at a first end of the crystalelement and a second portion having a different one of the plurality ofphotodetectors at the first end of the crystal element, and furtherwherein the plurality of internal defects are disposed along atransverse plane that delineates the first portion from the secondportion.
 8. The radiation detector of claim 7, wherein the plurality ofinternal defects define a shaped barrier.
 9. The radiation detector ofclaim 8, wherein the shaped barrier is triangular.
 10. The radiationdetector of claim 7, wherein the plurality of internal defects are moredensely spaced near the first end of the crystal element, and are moresparsely spaced away from the first end of the crystal element.
 11. Theradiation detector of claim 1, wherein the plurality of photodetectorscomprises a first array of photodetectors disposed on a first side ofthe scintillator block and a second array of photodetectors disposed ona side opposite the first side of the scintillator block.
 12. Theradiation detector of claim 1, further comprising a transparent lightguide disposed between the scintillator block and at least some of theplurality of photodetectors.
 13. A scintillator for a radiation detectorcomprising a monolithic block of scintillation material having aplurality of subsurface laser engraved defects configured to interactwith visible-wavelength photons produced in a scintillation eventoccurring in the scintillator.
 14. The scintillator of claim 13, whereinthe plurality of subsurface laser engraved defects define a grid ofinternal optical barriers that extend through a thickness of thescintillator.
 15. The scintillator of claim 14, wherein the grid ofinternal optical barriers define a regular array of scintillatorelements.
 16. The scintillator of claim 15, wherein the plurality ofsubsurface laser engraved defects further define a depth-of-interactionoptical boundary for each of the scintillator elements in the array ofscintillator elements, wherein the depth-of-interaction optical boundaryextends partially through the scintillator element along a plane. 17.The scintillator of claim 15, wherein the plurality of subsurface laserengraved defects further comprise a depth-of-interaction opticalboundary for each of the scintillator elements in the array ofscintillator elements, wherein the depth-of-interaction optical boundaryextends partially through the scintillator element along a plane, andfurther wherein the depth-of-interaction optical boundary varies indefect density along a length of the boundary.
 18. The scintillator ofclaim 15, wherein the plurality of subsurface laser engraved defects aredistributed throughout the monolithic block of scintillation material.19. The scintillator of claim 18, wherein the plurality of subsurfacelaser engraved defects are uniformly distributed throughout themonolithic block of scintillation material.